One of the world's worst 100 invasive alien species presently listed by the World Conservation Union, was discovered at the port of Nelson earlier this year. Described by some as the Genghis Khan of the ant world, the Argentine ant (Linepithema humile) has a wide dietary range and produces large numbers of aggressive and industrious workers, they are not poisonous, but do bite people. When established in optimal habitat, it often excludes all other ants, native and introduced.
An introduction perhaps not overly surprising considering 147 ant species worldwide have been recorded outside of their native habitat, with a disproportionately high number of transferred ants originating from the Neotropical and Oriental biogeographical regions, with the Pacific Islands being the recipients of the most transferred ant species.
South American native to Argentina and Brazil, the Argentine ants have now spread to more than 20 countries around the world, and were first found in New Zealand at the Mt Smart site of the 1990 Commonwealth Games in Auckland. And have since been found in Northland, Waikato, Bay of Plenty, Wellington, Chistchurch, and in March this year at the Port of Nelson.
Insect ecologist Dr Richard Harris of Landcare Research in Nelson, together with help from Port Nelson, the Tasman District Council and Nelson City Council, treated the port infestation which covered over 10 hectares. The operation generated media attention and raised community awareness, and to date no confirmed sightings have been reported from other Nelson locations. So have we eliminated them?
"No," Dr Harris explained, "what we have done is knock them back 99.9%, we have decimated the numbers around the areas we have treated and what we need to do next summer is go back and 'mop-up' the survivors."
"There were enormous numbers there in the first instance. The first stage is to knock them right back. Over the winter they get knocked back further as they don't like the cold all that much, and the numbers decline further. Next spring when the surviving queens start to produce more workers, building up in numbers and start foraging actively for food again - that's when we want to hit them again."
It appears that cold temperatures may save the native environment of the South Island from any invasion. Modelling work to predict where Landcare think the ant will get to in New Zealand reveal that much of the South Island is too cold outside of urban areas, with Nelson being right on the limit of where they would expect them to inhabit. "Urban areas provide warm buildings and concrete areas that are much warmer than the surrounding habitat. So in terms of the South Island we don't think they will be much of a problem outside urban areas," he added.
Dr Harris explained that wherever the Argentine ant establishes they displace everything else and if they spread throughout Nelson, this one would replace all other species that are currently a pest. There is no competition between individual Argentine ant colonies and they act as one super-colony, they think they are all related and share workers and queens, which leads to their being the support of much higher densities than other ant colonies would. This leads to more ants per unit area and a greater urban pest. There can be several nests per square metre and work overseas has shown colonies spreading out about 150m per year.
And Dr Harris pointed out the only good thing making them slightly easier to contain, is they don't have queens that fly. "A lot of ants, once you have them in New Zealand move around really rapidly and cover large distances with the queen having mating flights. But Argentine ants move either by us moving them with freight or pot plants, or by walking and a colony splitting off - a bunch of workers marching off with a queen to another site. So you have to remove the human transportation out of the loop."
The warmer top half of the North Island where there is a lot of coastal vegetation will be ideal for them Dr Harris said, and in these habitats they will displace all the native ants and impact on the rest of the invertebrate community. They will compete with birds for food and have been recorded killing nesting birds, going up trees in huge numbers and swarming over them.
"It is inevitable they are going to spread because they are in huge numbers in places like Auckland and there is no co-ordinated campaign to deal with them. There is the opportunity to try and keep them out of places like Nelson where they are localised at the moment."
"The main thing in the Nelson Marlborough area, is if you think you have an argentine ant is to drop a sample into us to confirm."
They are 2.5 mm long and a similar size to most other ants around, but they are a honey brown colour rather than black, and in very large numbers with strong trails coming into houses.
